It is customary in the Pulp & Paper Industry when producing chemical pulp to burn the used or residual liquor in a chemical and heat recovery unit. In a conventional kraft process chemical is recovered by injecting black liquor at a concentration of about 65% solids into the furnace and burning the organic material in the liquor in two stages. As the liquor enters the furnace, it is rapidly dried and forms a char bed above the hearth. This char bed burns under reducing conditions at a temperature of 1300.degree.-1700.degree. F. and the recovered chemicals are drained from the bed as a smelt and are discharged from the furnace. In the second stage of burning further oxygen is added to burn the gases generated in the char bed at a higher location in the furnace. Temperature in the second stage is generally about 2000.degree.-2100.degree. F. and the furnace is provided with suitable heat exchange means to recover heat and generate steam, thereby to supply the major portion of the steam requirements of the mill.
Such furnaces are susceptible to major safety problems as a result of serious explosions which occur when significant quantities of water contact the smelt. Although major efforts have been made to avoid such accidental contact, failure of a tube in the water wall or in the boiler section of the furnace, for whatever reason, has resulted in explosions.
It is also known to recover chemicals from residual liquors in the Pulp & Paper Industry using fluidized bed combustion units. In this case, the recovered inorganic chemicals are in the form of pellets and since the pellets are solid rather than molten as in the smelting furnace, the explosion hazard is eliminated. However, combustion must be carried out at a temperature of less than about 1300.degree. F. (for kraft liquor) to prevent the pellets from melting and aglomerating and defluidizing the bed. Generally, in existing units the low temperature is maintained by feeding a relatively dilute liquor to the bed so that the excess heat that could be used to generate steam is used in evaporating the extra liquid accompanying the solids i.e. the dilute liquor fed to the fluidized bed has a concentration in the range of 30-40% as compared to the 65% concentration in the conventional kraft furnace.
It is current practice in at least one kraft mill to burn a kraft liquor in a fluidized bed, thereby to generate pellets composed primarily of sodium carbonate and sodium sulfate and to inject these pellets onto the char bed of a conventional kraft recovery boiler thereby to reduce the pellets and increase the production of smelt i.e. to increase the recovery capacity of the mill by adding an oxidation stage to the recovery system. Such an arrangement is described in detail in U.S. Pat. No. 4,011,129 issued Mar. 8, 1977 to Tomlinson.
Fluidized bed recovery systems, whether used for kraft recovery or semi-chemical recovery have not normally been provided with heat exchange means, and thus, little, if any, heat has been recovered from these fluidized beds other than through waste-heat boilers or boiler sections provided in the freeboard area above the fluidized bed. However, it is well known in the burning of coal in fluidized beds to incorporate within the bed heat exchange means for the production of steam and it has also been proposed in certain applications to incorporate such heat exchange means in other fluidized bed chemical processes.
The cost of energy has increased significantly over the past few years and it has now become economically sound to recover heat from the burning of such residual liquor and it is desirable to maximize such heat recovery. Clearly the feeding of dilute residual liquors of 30-40% consistency reduces significantly the amount of heat that may be extracted from the fluidized bed. Similarly, the feeding of concentrated liquors of about 65% as used in the conventional kraft recovery furnace still incorporates a significant amount of water that must be evaporated in the furnace thus reducing the heat recoverable as steam. However, the 65% solids is about as high a solid content as one may obtain and still properly feed the concentrated liquor to the furnace.
It has also been proposed to concentrate residual liquor in a ventrui type scrubber fed with flue gases directly from the fluidized bed and to separate this concentrated liquor, mix it with ground solids obtained by grinding pellets extracted from the fluidized bed to form a slurry and to feed the resultant slurry back to the bed. The degree of fineness of the ground solids is used to obtain the desired particle size distribution in the bed. Such an arrangement is shown in Canadian Pat. No. 958,506 issued Dec. 3, 1974 to Heath.